1. Field of the Invention
The invention relates to a suction-air control device for controlling vacuum applied to suction openings of a printing press cylinder and, in particular, to such a device for opening and shutting off suction-air lines on sheet-transfer drums of sheet-fed rotary offset printing presses having sheet trailing-edge suction grippers.
Control devices of this general type serve for conveying small-format sheets of printing substrate on a variable-format cylinder provided with a group of suction openings constructed for the maximum format, aspiration or sucking-in of faulty air by the suction openings not covered by the smaller-format sheets of printing substrate being avoided. High energy consumption, poor efficiency, in an extreme case breakdown of the vacuum, aspiration or sucking-in of relatively large contaminant particles and increased noise emission during operation of the printing press are thereby avoided.
In the published German Patent Document DE-A-21 09 237, there is described a suction roller of adjustable suction width for sucking sheets or cut-to-size pieces of paper or the like onto it and conveying them, the suction roller having remote-controllable valves which serve for connecting individual groups of suction holes to a suction-air source. The valves are either fitted on control heads and can be operated from a central location or are arranged at a central location and are connected via pipelines to a plurality of control heads, but are in no way fitted on the cylinder. With this device, the suction widths of a plurality of rollers is able to be set within the shortest change-over times from a central location. A disadvantage thereof is that the construction of the disk or ring-shaped, wearing component of the control head becomes more complicated with increasingly finer graduation of the subdivision of the groups of suction holes and as a consequence requires more costly production. The device proposed in the prior art can be well utilized in the case of the cited envelope-making machines with broad graduation of the suction holes into three suction hole groups, but only poorly with printing-press cylinders requiring a finely graduated, greater number of suction holes and suction hole groups, respectively. Emphasis is placed upon solenoid valves as a particularly suitable possibility for controlling the valves. It is unfavorable, in this regard, that in addition to the pneumatic system already present in any event on the press, a second system be installed, the electrical system being prone to failure. Because the solenoid valves are comparatively expensive, their use in large numbers in the case of the printing-press cylinder with fine suction hole graduation, mentioned hereinbefore, is not very advantageous. The valves are not arranged on the suction roller, but rather, externally. The preferred solenoid valves also cannot be arranged well on the rotatable suction roller, because the electrical clutch would be complex. Due to the external valve arrangement, the suction-air channels leading from the valves to the suction openings on the lateral surface of the suction roller which are not covered by printing substrate or the like are exposed to a risk of soiling in the form of penetration or ingress by foreign bodies, for example accumulating particles of paper.
In German Published Non-prosecuted Patent Application (DE-OS) 27 54 370, a stacker roller is described having suction openings provided with automatically acting closing mechanisms which respectively close those suction openings which are not covered by covering material. These valves are pneumatically operated, but are not controllable, and instead closing and opening of their own accord. No pneumatic control device for the exactly timed feeding of the suction air in a manner dependent upon the angle of rotation is provided in this reference. It is unfavorable in this regard that the valves function comparatively unreliably. Reliable functioning is dependent upon the amount of vacuum applied to the suction openings. If printing substrates of different weights, for example thin paper and cardboard, are to be guided on a roller of such a construction, a variation in the intensity of the suction air is often required. A reduced intensity of the suction air may have as a consequence that the self-operated valves respond with delay, or more slowly. Contaminants deposited in the valves, for example lint, may moreover influence the sliding frictional conditions of the guided valve elements in an unpredictable manner. The guidance of printing substrate on a printing-press cylinder requires very exactly timed feeding of the suction air, however, and in particular absolutely reliable functioning of the valves with all types of printing substrate. This is particularly important, for example, if a sheet of printing substrate which is guided on a drum with suction openings holding the sheet is to be transferred to a downline drum, it being not always possible for the sheet to be simply drawn off from the sucking suction openings. As a result of valves not opening, or not opening at the correct time, it is possible, for example in the region of a corner of a sheet of printing substrate, for fluttering or creasing of the unsecured corner to occur. What is more, these effects may be misinterpreted by sensors on the printing press. A bent-over corner may be interpreted, for example, as a double sheet, as a consequence of which the sensor causes the press to stop when it may not be necessary. Regular cleaning, for example blowing out, of the valve seats after comparatively short servicing intervals would increase the downtime of the press and only partially remedy the disadvantage mentioned, because the unreliable functioning, as described hereinabove, is not only contamination-dependent but also construction-dependent. Integration of these self-operated valves into conventional rotary suckers would be a complex undertaking from a construction standpoint.
In German Published Non-prosecuted Patent Application (DE-OS) 41 26 643 A1, a drum for transporting and transferring sheets is described, which has shut-off valves for disconnecting the lines not required for paper format reasons. The shut-off valves are arranged on the drum and are not controllable but manually operatable.